The present invention relates to photovoltaic (PV) devices, and more particularly to a thin film molecular organic photovoltaic (OPV) device or solar cell.
Photovoltaic devices or solar cells have the potential to provide an unlimited source of energy by converting solar energy into electricity. Low-cost, lightweight and mechanically resilient solar power sources are of increasing interest for modern applications such as electronic textiles, synthetic skin and robotics. Compared with the inorganic photovoltaics, the primary benefits of organic photovoltaics (OPVs) are often listed as low-cost, low-weight, flexibility and compatibility with reel-to-reel processing for high volume production. However, due to the poor generation and extraction of electric charges, the power conversion efficiency (PCE) of OPVs is limited. The latest record of PCE is about 10.6% for OPVs, which is still much lower than that of inorganic solar cells. The relatively poor performance of OPVs originates from the low charge carrier mobility and small exciton diffusion length (˜10 nm) in organic semiconductor materials, which limit the thickness of active layers in OPVs, resulting in poor absorption of incident solar photons. It is a fundamental issue in OPVs to achieve efficient optical absorption for organic active layers much thinner than the optical absorption length. Therefore, in order to optimize both optical and electrical properties of OPVs simultaneously, light trapping strategies must be used to achieve high optical absorption in the ultrathin organic active layers.
On the other hand, transparent conducting electrodes (TCEs) are essential components in many optoelectronic devices, including LEDs, electronic displays, and photovoltaics. Most OPVs employ an indium tin oxide (ITO) electrode due to ITO offers transparency at visible and near-IR frequencies as well as good electrical conductivity. However, ITO electrodes are not the optimum choice for OPVs and suffer from several drawbacks, such as collateral damage to the underlying organic materials, poor mechanical stability, and the band structure of ITO can hinder efficient photocurrent generation. In addition, the limited source of indium and the increasing demand for ITO from the rapidly growing visual display market have drastically increased the cost of ITO, highlighting a growing need for an alternative low-cost TCE for OPVs.
An improved OPV solar cell is therefore desired.